In a radio communication system in which a signal is radio-transmitted from a transmitting device to a receiving device, the transmitting device transmits the signal by using a power amplifier. The power amplifier may have a nonlinear amplification characteristic, that is, a characteristic in which a relation between an input signal and an output signal is nonlinear. The nonlinear characteristic of the power amplifier may cause Intermodulation Distortion (IMD) in the signal transmitted by the transmitting device. The IMD may cause Intersymbol Interference (ISI) and Interchannel Interference (ICI). Therefore, in the receiving device that receives a signal that is distorted due to the nonlinear characteristic of the power amplifier, a reception quality such as a Bit Error Rate (BER) may deteriorate.
To address the above-described problem, a predistortion method as a method for linearizing the nonlinear characteristic of the power amplifier is performed. For the predistortion method, a predistorter is provided as a linearizer in the transmitting device. The predistorter adds distortion with a reverse characteristic of the amplification characteristic of the power amplifier to a signal before the signal is input into the power amplifier. By transmitting the signal added with the distortion of the reverse characteristic through the power amplifier, the nonlinear distortion of the amplified signal with respect to the signal before the signal is input into the predistorter is suppressed. The nonlinear distortion caused by the power amplifier may be compensated in the transmitting device by using the predistorter.
In the power amplifier, nonlinear distortion may be caused by a so-called memory effect. Regarding the memory effect, the power amplification corresponding to a symbol is influenced by a symbol that is input earlier than this symbol. The memory effect may be generated by influence such as a filter delay, a frequency response of a matched circuit, nonlinear capacitance, a response speed of a bias network, for example. To address the above-described problem, there is a proposal of a power amplifier that compensates the nonlinear distortion caused by the memory effect (for example, see International Publication Pamphlet No. 2007/004252).
Regarding the receiving device used in the radio communication system, there is a turbo equalization technique for detecting data before the data is decoded by repeating processing by an equalizing unit that compensates the distortion and the decoding unit that performs error correcting processing (see, for example, Japanese Laid-open Patent Publication No. 2009-188640).
For the method for compensating the distortion caused by the memory effect of the power amplifier, the effectiveness of distortion compensation may be further improved. That is, calculation of the reverse characteristic of the amplification characteristic of the power amplifier with accuracy is not easy, so that the accuracy of predistortion is unlikely to be improved. Compared to predistortion in consideration of the memory effect, a predistortion algorithm in consideration of the memory effect is more complicated than the predistortion algorithm without consideration of the memory effect, so that the circuit of the predistorter is complicated.